Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
RUCOTE 103 polymer (Ruco) technical bulletins describe increasing hardness by blending RUCOTE 107 with a high hydroxyl number resin RUCOTE 103 to provide coatings having improved hardness.
U.S. Pat. No. 5,229,470 describes low gloss polyurethane powder coatings prepared by blending hydroxyl polyesters having a difference in gel time of at least three minutes.
U.S. Pat. No. 3,211,585 describes polyesters comprising the reaction product of a polymeric ester of a polycarboxylic acid of the group consisting of terephthalic acid, isophthalic acid and tris(hydroxyethyl)isocyanurate, with 10-40 % of an organic polyisocyanate which is useful for coating electrical conductors.
U.S. Pat. No. 3,249,578 describes polyesters prepared from tris(hydroxyethyl)isocyanurate used to coat electrical conductors.
U.S. Pat. No. 3,646,374 describes polyester-imide resins comprised of tris(hydroxyethyl)isocyanurate used for coating and electrically insulating core members of dynamoelectric machines.
U.S. Pat. No. 4,476,279 discloses a magnet wire enamel comprising a high solids low viscosity organic solvent solution of a tris(hydroxyethylisocyanurate)polyester having a hydroxyl number of 216 to 316, and a ratio of hydroxyl to carboxyl groups of 1.65 to 2.0:1.
The present invention, as described below, relates to novel polyurethane powder coatings exhibiting high hardness upon cure and good caking stability during storage. Prior to the present invention, polyurethane coatings having high hardness were prepared by mixing two types of polyester resins having low and high hydroxyl numbers. This is generally accomplished by preparing resins having high hydroxyl numbers due to increased use of a polyol such as trimethylolpropane; however, this approach lowers the Tg of the composition and hence detracts from the caking stability.
It has now been found that a polyester resin with a high hydroxyl number and high Tg can be made based on isophthalic acid and tris(hydroxyethyl)isocyanurate. This resin can be blended with polyester resins having low hydroxyl numbers to produce powder coating compositions having good caking stability and coatings having high hardness after application by electrostatic spraying and curing.